Epoxy adhesive compositions are reaction adhesive compositions comprising an epoxy resin, a curing agent and usually a latent accelerator. Upon heating, the epoxy groups of the epoxy resin react with the curing agent linking the epoxy resin compounds by a polyaddition reaction to obtain a cured product.
Such a cured product is known to have good mechanical properties and a corrosion resistance superior to the cured product of other reaction adhesives. These characteristics make epoxy based adhesive compositions particularly useful for demanding applications where stringent mechanical requirements must be satisfied, for example, in the automotive industry. Adhesives for bonding parts of the body structure of a vehicle, for example, a car, a lorry (truck), a bus or a train, are called structural adhesives.
Generally, heat-curable epoxy adhesives are used as structural adhesives.
In the manufacturing process of a vehicle, the heat-curable structural adhesive which has been applied to the body-in-white structure is subjected to spray-wash or other cleaning methods followed by phosphatizing and e-coating prior to the final heat-curing of the structural adhesive in the e-coat oven. In order to prevent the heat-curable structural adhesive from being washed off, it is conventionally pre-cured by thermal methods such as inductive pre-curing or by using a body-in-white oven. However, such an additional pre-curing step makes the manufacturing process laborious. Additionally, body-in-white ovens are expensive.
As an alternative to the heat-curable epoxy adhesives, the use of two-component epoxy adhesives as structural adhesives has been considered. Two-component epoxy adhesives comprise two separate components, the first comprising the epoxy resin and the second comprising the epoxy reactive compound. Such two-component epoxy adhesives generally cure immediately after mixing the two components and do not require the equipment used for the pre-curing and the curing of heat-curable epoxy adhesives.
A number of two-component epoxy adhesives have been described in the past.
U.S. Pat. No. 4,728,737 discloses a two-component structural adhesive comprising an epoxy resin component and a hardener component (incorporated herein by reference).
U.S. Pat. No. 4,916,187 relates to a two-component epoxy resin adhesive composition consisting of a first polyepoxide component and a second curing component comprising a finely dispersed solid salt of a polyamine and a polyphenol in a liquid adduct (incorporated herein by reference).
U.S. Pat. No. 5,629,380 discloses a two-component epoxy adhesive comprising a first component including an epoxy catalyst and an amine curing agent and a second component including an epoxy resin having a functionality of more than 1 (incorporated herein by reference).
US 2002/0164485 and U.S. Pat. No. 6,577,971 both relate to a two-component epoxy adhesive composition comprising a first component including an epoxy resin and a silane coupling agent and a second component including an aliphatic amine and a polyamine (incorporated herein by reference).
U.S. Pat. No. 6,248,204 discloses a room-temperature curable epoxy resin composition comprising a first component including an epoxy resin and a second component including an amine-based hardener (incorporated herein by reference).
The above-mentioned compositions have however several disadvantages which limit their use as structural adhesives. Their pot life, i.e., the time available for using the epoxy adhesive composition after the components are mixed, is short since the curing starts at room temperature immediately after mixing. Thus, the assembly of the parts to be bonded together has to be carried out shortly after application of the epoxy adhesive. In addition, said two-component epoxy adhesive compositions lead to cured products which do not meet the requirements generally demanded of structural adhesives. In particular, the crash stability of the cured product of said two-component epoxy adhesives does not comply with the stringent requirements for this application. Generally, the glass transition temperature of a cured two-component epoxy adhesive is low. Their oil uptake is poor which is particularly disadvantageous in the manufacture of vehicles where oily steels are bonded together.
The object of the present invention is to provide a two-component structural adhesive which is wash-off resistant without thermal pre-curing, which has an assembly time of several hours, can be stored in the uncured state up to several weeks and which results in a cured product having a high crash stability, a high glass transition point and a good oil uptake.